Precision high-frequency capacitor formed on semiconductor substrate

ABSTRACT

A precision high-frequency capacitor includes a dielectric layer formed on the front side surface of a semiconductor substrate and a first electrode on top of the dielectric layer. The semiconductor substrate is heavily doped and therefore has a low resistivity. A second electrode, insulated from the first electrode, is also formed over the front side surface. In one embodiment, the second electrode is connected by a metal-filled via to a layer of conductive material on the back side of the substrate. In alternative embodiments, the via is omitted and the second electrode is either in electrical contact with the substrate or is formed on top of the dielectric layer, yielding a pair of series-connected capacitors. ESD protection for the capacitor can be provided by a pair of oppositely-directed diodes formed in the substrate and connected in parallel with the capacitor. To increase the capacitance of the capacitor while maintaining a low effective series resistance, each of the electrodes may include a plurality of fingers, which are interdigitated with the fingers of the other electrode. The capacitor is preferably fabricated in a wafer-scale process concurrently with numerous other capacitors on the wafer, and the capacitors are then separated from each other by a conventional dicing technique.

This application is a divisional application of U.S. application Ser.No. 09/661,483, filed Sep. 14, 2000, now U.S. Pat. No. 6,538,300, issuedMar. 25, 2003.

CROSS-REFERENCE TO RELATED APPLICATION

This invention is related to application Ser. No. 09/545,287 by Kasem etal., filed Apr. 7, 2000, entitled “Vertical Structure And Process ForSemiconductor Wafer-Level Chip Scale Packages”, which is incorporatedherein by reference in its entirety.

FIELD OF THE INVENTION

This invention relates to semiconductor technology and in particular tothe formation of a high-frequency capacitor on a semiconductorsubstrate.

BACKGROUND OF THE INVENTION

Higher frequencies are increasingly being used in communicationstechnology. For example, frequencies in the range of 450 MHz to 3 GHzare used in cellular communications and frequencies in the range of 10GHz to 18 GHz are used in satellite video and data transmission.

These applications require small, precise capacitors. Multi-layerceramic capacitors have been employed for this purpose, but they tend tobe lacking in precision and performance. Thin film capacitors haveimproved precision and performance but they are expensive.

Accordingly, there is a need for a precision high-frequency capacitorthat can be manufactured at a reasonable cost.

SUMMARY OF THE INVENTION

In accordance with this invention, a precision high-frequency capacitoris formed on a heavily-doped semiconductor substrate having first andsecond principal surfaces. The capacitor includes a dielectric layer onthe first principal surface of the substrate and a main electrode layeron the dielectric layer. A conductive layer is formed on the secondprincipal surface of the substrate. A via containing a conductivematerial extends through the substrate. A second electrode layer isformed over the first principal surface of the substrate, adjacent anopening of the via. The second electrode is electrically connected tothe conductive layer by means of the conductive material in the via.Thus, when a voltage difference is applied to the electrodes, the mainelectrode layer and the substrate act as the “plates” of the capacitor,separated by the dielectric layer.

In an alternative embodiment, the via is omitted, and the secondelectrode layer, electrically insulated from the first electrode layer,is formed over the first principal surface of the substrate. In oneversion, the second electrode is separated from the substrate by thedielectric layer, creating in effect a pair of series-connectedcapacitors, with the substrate representing the common terminal betweenthe capacitors. In another version, the second electrode is inelectrical contact with the substrate, creating a single capacitor. Eachof the electrode layers may include a plurality of fingers, with thefingers being interdigitated. The dielectric layer, often an oxide, maybe thinner under the fingers than under the “palm” portions of theelectrode layers from which the fingers protrude.

Capacitors in accordance with this invention exhibit numerous advantagesas compared with prior art capacitors. They can be fabricated at a waferlevel with a very low effective series resistance (ESR). They canfunction at very tight tolerances (e.g., <2%) throughout theiroperational range and can operate at very high frequencies (e.g., up to5 GHz and higher). They can have a quality (Q) factor, for example, thatis much higher than 1000 at 1 MHz.

BRIEF DESCRIPTION OF THE DRAWINGS

This invention will be best understood by reference to the followingdrawings, in which like components have the same reference numeral. Thedrawings are not necessarily drawn to scale.

FIG. 1 is a cross-sectional view of a capacitor in accordance with theinvention containing a via through the substrate.

FIG. 1A is a graph showing the Q of a capacitor in accordance with thisinvention, i.e., the ratio of the imaginary part of the impedance to thereal part of the impedance, as a function of frequency.

FIGS. 2A-2J illustrate the steps of a process that can be used tofabricate the capacitor of FIG. 1.

FIG. 3 is a cross-sectional view of a capacitor in accordance with thisinvention containing two electrodes on the same surface of thesubstrate.

FIG. 4 is a cross-sectional view of a capacitor with trenches formedunder each of the electrodes.

FIG. 5 is a cross-sectional view of a capacitor similar to the capacitorshown in FIG. 3 except that one of the electrodes is electricallyconnected to the substrate.

FIG. 6 is a top view of a capacitor wherein the electrodes have fingersinterdigitated with each other.

FIG. 7 is a cross-sectional view of the capacitor shown in FIG. 6showing that the dielectric layer is thinner under the fingers.

FIG. 8 is a circuit diagram of an ESD-protected capacitor containing apair of oppositely-directed diodes.

FIG. 9 is a cross-sectional view of an ESD-protected capacitor inaccordance with this invention.

FIGS. 10a and 10 b are graphs showing simulated breakdowncharacteristics of an ESD-protected capacitor of the kind shown in FIG.9.

FIG. 11 is a graph showing the simulated effective capacitance of theESD-protected capacitor.

DESCRIPTION OF THE INVENTION

The principles of this invention will be described by reference to thefollowing embodiments, which are illustrative only.

FIG. 1 shows a cross-sectional view of a first embodiment according tothe invention. Capacitor 10 is formed on an N+ silicon substrate 102.Substrate 102 may be doped to a concentration of 3 to 4×10¹⁹ cm⁻³, forexample, and may have a resistivity of about 2 mΩ-cm and as high asabout 3 mΩ-cm. A dielectric layer 104 is formed on the front surface ofsubstrate 102. Dielectric layer 104 is formed of SiO₂, which may bethermally grown or deposited by chemical vapor deposition (CVD).Alternatively, layer 104 could be formed of another dielectric such as anitride or a combination of an oxide and a nitride. A thermally-grownoxide is reliable and reproducible and can withstand electric fields upto 4 MV/cm without deterioration. The 3σ variability of the thickness ofa thermally-grown oxide thicker than 0.1 μm is less than 1.5%.

On top of dielectric layer 104 is a main electrode 106 and a secondelectrode 108. Electrodes 106 and 108 can be a single- or multi-layerstructure, and can be made of doped polysilicon, a refractory metal, arefractory metal suicide, an aluminum-based alloy, copper or combinationof the foregoing materials. If they are formed of metal, electrode 106may include a “seed” or “barrier” layer of a metal (e.g., Ta/Cu)deposited on substrate 102 by sputtering or evaporation, overlain by aplated layer. Electrodes 106 and 108 are covered by an insulatingpassivation layer 110. Openings are formed in passivation layer 110, andsolder balls 112 and 114 are deposited the openings to allow electricalcontact to be made to the electrodes 106 and 108.

Beneath the second electrode 108, a via or through-hole 116 is formedthrough N+ substrate 102. A conductive material 118 such as aluminum orcopper fills the via 116. The conductive material 118 contacts aconductive layer 120 which is formed on the back side of substrate 102.Conductive layer 120 may include a metal seed layer deposited onsubstrate 102 by sputtering or evaporation, overlain by a plated metallayer.

Capacitor 10 thus includes a first “plate” represented by main electrode106, which is contacted via solder ball 112; and a second “plate”represented by N+ substrate 102, which is contacted via solder ball 114,second electrode 108, conductive material 118 and conductive layer 120.The “plates” are separated by dielectric layer 104.

The thickness of dielectric layer 104 can be in the range of 50 Å to 2μm. The thinner dielectric layer 104 is, the higher the capacitance. Onthe other hand, the thinner dielectric layer 104 is, the lower themaximum voltage that capacitor 10 can be exposed to without damagingdielectric layer 104. For example, if dielectric layer 104 is an oxidehaving a thickness of 0.1 μm, capacitor 10 would have a capacitance ofroughly 350 pF/mm².

Silicon substrate 102 can have a thickness of 200 μm or less. Dopingsubstrate 102 to a concentration higher than 1×10¹⁹ cm⁻³ keeps theeffective series resistance (ESR) at a low level and avoids theformation of a depletion layer in the substrate. For example, the ESRfor a silicon substrate doped to a concentration of 2×10¹⁹ cm⁻³ was only2.4 mΩmm².

In addition, it is desirable that the Q factor of the capacitor behigher than 1000 at 1 MHz. The Q factor is defined by the followingequation: $Q = \frac{X_{C}}{R_{S}}$

where X_(C) is the impedance and R_(S) is the series resistance of thecapacitor at a particular frequency.

FIG. 1A is a plot of X_(C) and R_(S) as a function of frequency forcapacitor 10, described above, wherein the thickness of the oxidedielectric layer 104 is 0.1 μm and the N+ silicon substrate is doped to2×10¹⁹ cm⁻³. As shown, the Q factor of the capacitor is higher than 100up to a frequency of about 2 GHz and is greater than 1000 at 100 Mhz.

While capacitor 10 can be fabricated by a number of processes, FIGS.2A-2J illustrate the steps of one process that may be used.

As shown in FIG. 2A, the process begins with N+ silicon substrate 102.Preferably, substrate 102 is one die of a wafer that will be separatedfrom the other dice at the completion of the process. Substrate 102 mayor may not include an epitaxial layer.

Dielectric layer 104 is formed by growing an oxide (SiO₂) layerthermally on the front (top) surface of substrate 102. For example, a0.2 μm thick oxide layer can be grown by heating the substrate to 1100°C. for 6 minutes in a wet atmosphere.

Referring to FIG. 2B, a barrier layer 202 of Ta/Cu is sputtered over theentire surface of oxide layer 104. Layer 202 can be 0.5 to 1.0 μm thick,for example. A photoresist layer 204 is deposited and patterned as shownin FIG. 2B to define where the main electrode will be located.

A copper layer 206 is plated onto the exposed portions of Ta/Cu layer202, and photoresist layer 204 is removed, leaving the structure shownin FIG. 2C.

The front side of substrate 102 is then taped or otherwise supported,and substrate 102 is thinned from the back side. Substrate 102 may bethinned by grinding its back side. Alternatively, other thinningtechniques such as wet etching and vacuum plasma etching can be used tothin substrate 102. Another possibility is the atmospheric downstreamplasma (ADP) plasma etching system available from Tru-Si Technologies,Inc. of Sunnyvale, Calif. Substrate 102, which can initially be in therange of 625 μm thick, can be thinned to a thickness of less than 200μm, for example.

After the thinning process has been completed, the tape or other supportis removed. A layer 208 of Ta/Cu is sputtered or evaporated over theentire back side surface of substrate 102, and a copper layer 210 isplated onto Ta/Cu layer 208, leaving the structure shown in FIG. 2D.Copper layer 210 can be 2-3 μm thick, for example.

As shown in FIG. 2E, a photoresist layer 212 is deposited on the frontside of silicon substrate 102. Photoresist layer 212 is patterned andetched to produce an opening 214. A conventional wet etch process can beused, for example. Silicon substrate 102 is etched through opening 214to form a via 216 and thereby expose the surface of barrier layer 208.As shown in FIG. 2E, via 216 is conical in shape because silicon etchesalong oblique planes. Depending on the shape of opening 214, via 216could be any shape.

As shown in FIG. 2F, photoresist layer 212 is then removed, and a seedlayer 218 of Ta/Cu is sputtered onto the entire front side surface ofthe structure. Ta/Cu layer 218 can be 0.5-1.0 μm thick, for example.

As shown in FIG. 2G, a photoresist layer 220 is deposited and patterned,leaving a portion of the Ta/Cu layer 218 in the vicinity of the via 216exposed.

As shown in FIG. 2H, a copper layer 222 is plated onto the exposedportions of Ta/Cu layer 218, filling via 216 and overflowing onto thesurface of substrate 102.

As shown in FIG. 2I, photoresist layer 220 is removed and Ta/Cu layer218 is etched, leaving the copper layer 222 in place.

As shown in FIG. 2J, a passivation layer 224 is formed and patternedover the surface of the structure by screen printing, with openings thatexpose portions of copper layers 206 and 222. Solder bumps 226 and 228are formed on the exposed portions of copper layers 206 and 222. Theresult is capacitor 10 shown in FIG. 1, which can be mounted on aprinted circuit board (PCB) or other structure, using flip-chip mountingtechniques. Optionally, a second passivation layer 230 can be formed onthe back side of the structure.

Capacitor 10 is preferably formed along with other similar capacitors ona single wafer. If so, following the fabrication of the capacitors, thedie which contains capacitor 10 is separated from the other dice in thewafer by sawing the wafer along the scribe lines.

FIG. 3 shows a cross-sectional view of an alternative embodiment.Capacitor 30 is actually a pair of capacitors connected in series. Adielectric layer 302 is formed on N+ silicon substrate 102. Substrate102 could have a doping concentration of 2×10¹⁹ cm⁻³, for example, andlayer 302 could be thermally-grown oxide with a thickness of 0.1 μm. Ametal layer is deposited on dielectric layer 302 and is patterned, usingnormal photolithographic processes, to form a first electrode 304 and asecond electrode 306. A passivation layer 308 is deposited on the topsurface of the structure. Openings are formed in passivation layer 308,and solder balls 310 and 312 are formed as described above.

While capacitor 30 is simpler and less expensive to fabricate than, forexample, capacitor 10 (FIG. 1), its capacitance is lower and its seriesresistance is greater. For example, the effective capacitance per unitarea is up to 4 times smaller than that of a vertical structure. Theseries resistance increases with the square of the lateral dimension ofthe device.

The capacitance per unit area of capacitor 40, shown in FIG. 4, issignificantly increased by the formation of trenches 406 underelectrodes 304 and 306. Dielectric layer 402 extends into trenches 304and 306 and lines the walls thereof in the manner of a normaltrench-gated MOSFET. Trenches 304 and 306 are filled with a conductivematerial 404, such as polysilicon, which is in electrical contact withelectrodes 304 and 306. The net result is to increase the effective areaof the interface between the “plates” and the dielectric layer of thecapacitor.

FIG. 5 shows a cross-sectional view of a capacitor 50 which is similarto capacitor 30 shown in FIG. 3 except that the electrode 504 is inelectrical contact with the N+ substrate 102. Electrode 502 is separatedfrom substrate 102 by a dielectric layer 506 having a defined thickness.Capacitor 50 has a capacitance value per unit area similar to that ofcapacitor 10 shown in FIG. 1. However, the lateral placement ofelectrodes 502 and 504 leads to a larger effective series resistance(ESR) which is a function of the lateral dimension of the device.

FIG. 6 is a top view of a capacitor 60 in which the first electrode 602has fingers 602 a-602 c that are interdigitated with fingers 604 a-604 dof the second electrode 604. FIG. 7 is a cross-sectional view taken atcross-section 7—7 shown in FIG. 6 (note that the scales of FIGS. 6 and 7are not the same). In an active region 606, where the fingers areinterdigitated, a thin dielectric layer 618 is formed over thesubstrate. A relatively thick dielectric layer 614 separates theremaining “palm” portion of electrode 602 from N+ substrate 102, and arelatively thick dielectric layer 616 separates the palm portion ofelectrode 604 from substrate 102.

The capacitance of capacitor 60 is determined by the number anddimensions of the fingers. As indicated in FIG. 6, electrode 604 can beseparated from the N+ substrate by a dielectric layer, creating a pairof capacitors, or it can be in direct electrical contact with the N+substrate (in the manner of electrode 504 in FIG. 5), creating a singlecapacitor. In many embodiments, the pitch “p” of the finger layout willbe less than 300 μm. For example, a capacitor 60 wherein electrode 604is in direct electrical contact with the substrate and the pitch of theinterdigitated fingers is 250 μm (finger width of 200 μm, spacing of 50μm) and the thin dielectric layer 618 is a 0.1 μm-thick oxide layer, hasa capacitance of 150 pF/mm² and an ESR of 12 mΩmm².

The thin dielectric layers used in precision capacitors make thesedevices very susceptible to damage from electrostatic discharges (ESDs).For example, ESDs can be generated by handling during the assemblyprocess. One way to protect against ESDs is to connect a pair ofoppositely-directed Zener diodes D1, D2 in parallel with the capacitor,as shown in the circuit diagram of FIG. 8. When an ESD voltage spikeoccurs, one of the diodes conducts in the forward direction and theother diode breaks down at a predetermined voltage and conducts in thereverse direction, thereby providing a current path around thecapacitor. The voltage at which current flows in the protective path isequal to the reverse breakdown voltage of one diode plus the voltage atwhich the other diode conducts in the forward direction (typically about0.5 V). (As used herein, the term “oppositely-directed” means that thediodes are connected in series with either their anodes facing eachother or their cathodes facing each other, such that any current in theseries path flows through one of the diodes in the forward direction andthrough the other diode in the reverse direction.)

In accordance with an aspect of this invention, the protective diodescan be formed in the substrate itself, as shown in the ESD-protectedcapacitor arrangement of FIG. 9. An N+ region 902, a P region 904 and anN+ region 906 are formed in substrate 102 beneath electrode 106. Theregions are formed such that there is a first PN junction between N+region 902 and P region 904 that represents one of the diodes and asecond PN junction between P region 904 and N+ region 906 thatrepresents the other diode. The doping concentrations of regions 902,904 and 906 are set such that the PN junctions break down in the reversedirection at a desired voltage. The breakdown voltage depends on thedoping concentration on the more lightly doped side of the PN junctionand other factors that are well-known in the art. See, for example, Sze,Physics of Semiconductor Devices, 2^(nd) Ed., John Wiley & Sons (1981),pp. 99-108, which is incorporated herein by reference.

The second N+ region 906, which extends into the P region 904 as well asthe N+ substrate, is used to provide a symmetrical breakdowncharacteristic of the diode pair. In some embodiments, N+ region 906 maybe omitted.

To maintain the high RF performance capabilities of the capacitor, theimpedance of the Zener diodes can be set at a level that is higher thanthe capacitor by a factor of 1000 or more.

Processes for forming the diodes in the substrate are well known tothose skilled in the art. One such process is as follows:

1. Initially, an N-type epitaxial (epi) layer that is 2.5 μm thick isformed on the top surface of the substrate. The doping concentration ofthe epi layer is 1×10¹⁶ cm⁻³, far less than that of the underlyingportion of the substrate.

2. A first photoresist mask with an opening defining the active areawhere the capacitor will be located is formed over the epi layer, andphosphorus is implanted through the opening in the mask at a dose of8×10¹⁵ cm⁻² and an energy of 80 keV to set the doping concentration ofthe epi layer to approximately the same level as the rest of the N+substrate (10¹⁹ cm⁻³). The first mask is then removed.

3. After the phosphorus implant into the active area through the firstmask, another mask is formed over the substrate with an opening definingwhere P region 904 will be located. Boron is implanted through theopening in this mask, for example, at a dose of 2×10¹³ cm⁻² and anenergy of 80 keV, to form P region 904.

4. The substrate is annealed at 1150° C. for 30 minutes to drive thephosphorus and boron implants through the epi layer.

5. The oxide dielectric layer 104 is thermally grown as described above.

6. After the oxide layer has been grown, a third photoresist mask isformed on the oxide layer and patterned to create openings which definethe N+ regions 902 and 906.

7. The oxide layer is partially etched through the openings in the thirdphotoresist mask to avoid the need to implant dopant through a thickoxide film.

8. Phosphorus is then implanted through the openings in the third maskand the thinned oxide layer at, for example, a dose of 3×10¹⁵ cm⁻² andan energy of 60 keV to form N+ regions 902 and 906.

9. The third photoresist mask is removed, and a blanket boron implant isperformed through the oxide layer to set a surface doping of the P-well.This can be done, for example, at a dose of 3×10¹² cm⁻² and an energy of60 keV. The boron dopant can be activated by annealing at 950° C. for 30minutes in an oxidizing ambient.

10. A fourth photoresist mask is formed and patterned with an openingover the area where contact is to be made to the N+ region 902. Theoxide layer is etched through the opening to expose N+ region 902. Thefourth mask is then removed.

Following this, the process described above continues with the formationof the electrodes 106 and 108.

Numerical simulations were done to calculate the performance of theESD-protection structure shown in FIG. 9. The dimensions of thestructure were as follows:

Width (W1) of P region 904:  5 μm Width (W2) of N+ region 902:  3 μmLength of structure: 100 μm

FIG. 10a shows the IV characteristic of the structure with electrode 106biased positive with respect to electrode 114 (“accumulation bias”), andFIG. 10b shows the IV characteristic of the structure with electrode 106biased negative with respect to electrode 114 (“depletion bias”). Asindicated, the diode pair breaks down in the range of 16-19 V in eitherdirection. FIG. 11 shows that the effective capacitance of the combinedcapacitor and ESD-structure remains quite constant at about 0.15 pFthroughout the frequency range from 0.1 to 10 GHz.

The embodiments of this invention described above are only illustrative,and not limiting. Numerous alternative embodiments will be apparent topersons skilled in the art from the above description.

We claim:
 1. A precision high-frequency capacitor comprising: aheavily-doped semiconductor substrate having a first principal surfaceand a second principal surface; a first dielectric layer portion formedon the first principal surface of the semiconductor substrate; a firstelectrode layer formed on the first dielectric layer portion andelectrically insulated from the semiconductor substrate; a seconddielectric layer portion formed on the first principal surface of thesemiconductor substrate; and a second electrode layer formed on thesecond dielectric layer portion, the second electrode layer beingelectrically insulated from the semiconductor substrate and the firstelectrode layer.
 2. The precision high-frequency capacitor of claim 1wherein the doping concentration of the semiconductor substrate isgreater than 1×10¹⁹ cm⁻³.
 3. The precision high-frequency capacitor ofclaim 1 wherein the first dielectric layer portion and the seconddielectric layer portion comprise an oxide.
 4. The precisionhigh-frequency capacitor of claim 1 wherein the thickness of the firstdielectric layer portion and the second dielectric layer portion isgreater than or equal to 0.005 micron.
 5. The precision high-frequencycapacitor of claim 1 further comprising a passivation layer overlyingthe first electrode layer and the second electrode layer, a firstopening being formed in the passivation layer over the first electrodelayer and a second opening being formed in the passivation layer overthe second electrode layer.
 6. The precision high-frequency capacitor ofclaim 5 comprising a first metal ball in the first opening and a secondmetal ball in the second opening, the first metal ball beingelectrically connected to the first electrode layer, the second metalball being electrically connected to the second electrode layer.
 7. Theprecision high-frequency capacitor of claim 1 comprising a trench in thesubstrate beneath the first dielectric layer portion, the firstdielectric layer portion extending along the walls of the trench, thetrench comprising a conductive material, the conductive material beingin electrical contact with the first electrode layer.
 8. The precisionhigh-frequency capacitor of claim 7 wherein the first dielectric layerportion and the second dielectric layer portion comprise an oxide. 9.The precision high-frequency capacitor of claim 7 wherein the conductivematerial comprises polysilicon.
 10. The precision high-frequencycapacitor of claim 7 further comprising a passivation layer overlyingthe first electrode layer and the second electrode layer, a firstopening being formed in the passivation layer over the first electrodelayer and a second opening being formed in the passivation layer overthe second electrode layer.
 11. The precision high-frequency capacitorof claim 10 comprising a first metal ball in the first opening and asecond metal ball in the second opening, the first metal ball beingelectrically connected to the first electrode layer, the second metalball being electrically connected to the second electrode layer.
 12. Theprecision high-frequency capacitor of claim 1 wherein the firstelectrode layer comprises a first plurality of fingers and the secondelectrode layer comprises a second plurality of the fingers, the firstplurality of fingers and the second plurality of fingers beinginterdigitated.
 13. The precision high-frequency capacitor of claim 12wherein the first plurality of fingers extend from a first palm portionof the first electrode layer, the first dielectric layer portion beingthinner under the first plurality of fingers than under the first palmportion.
 14. The precision high-frequency capacitor of claim 12 whereinthe second plurality of fingers extend from a second palm portion of thesecond electrode layer, the second dielectric layer portion beingthinner under the second plurality of fingers than under the second palmportion.